The invention relates to the manufacturing of multi lenses optical modules, such as, but not limited to, the optical modules used in the camera for mobile phones.
The invention relates in particular to a method and device for aligning a lens with an optical system, the optical system comprising for example one lens or more. The invention relates more particularly to a method and device for implementing a fast coarse pre-alignment of the lens with the optical system. The coarse pre-alignment allows to subsequently rapidly conduct a precise alignment of the lens with the optical system.
As a preamble, it is to be noted that the definition of the terms “focal distance”, “hyperfocal distance”, “convergence”, “optical axis”, “sagittal”, “tangential”, “MTF or Modulation Transfer Function”, “optical axis”, “spatial frequency”, “optical center” can be found in:    Warren J. Smith. Modern Optical Engineering. McGraw-Hill.
FIG. 1 shows an elevation view of the main elements composing an optical module 10 that can be used in a camera of a mobile phone. A support 12 comprises a cylindrical recess 14 having an axis 16 and a base 18. Base 18 comprises an aperture around axis 16. A number of lens assemblies 20a, 22a; 20b, 22b, 20c, 22c, 20d, 22d (four are illustrated) comprising each a spacer ring (20a, 20b, 20c, 20d) followed by a lens (22a, 22b, 22c, 22d) are maintained within the recess 14 with a cylindrical barrel 24 that comprises an aperture around axis 16. A sensor 26 is eventually provided such that it receives light crossing the lenses along axis 16. Sensor 26 can form part of the module or can be provided separately.
The quality of an optical module such as module 10 depends strongly on the alignment of the optical axis of the different lenses (22a, 22b, 22c, 22d) composing the module.
In the case of the modules used in mobile phones, the focal length of the module, which is the distance from the optical center of the lens assembly to the sensor onto which is formed the image, is constraint by the small dimension of the phone, and must generally range from 1 mm to 6 mm. Such a short focal distance is obtained by using a lens assembly combining several lenses having a high convergence, ranging approximately from 100 to 1000.
The higher the convergence of a given lens, the more important is the alignment of the optical axis of this lens with the optical axis of the rest of the module. It follows that for modules to be used in high resolution camera such as comprising above 5 megapixels with a focal length in the range of 3 to 5 mm, the alignment of the lenses has to be done with an accuracy of one micron, or less.
High quality modules composed of more than 3 lenses currently require at least one alignment made with an accuracy of 1 to 5 microns. Making such alignments is very time consuming.
A need exists for a method and device that allow aligning rapidly and with great precision a lens with an optical system.